The approaches to the problem of cellular resistance to virus infections which are being proposed in this application are (a) studies of cells persistently infected with Newcastle disease virus and an evaluation of the role of temperature-sensitive mutants in the establishment and maintenance of persistent infections with this and other viruses; (b) investigation of factors influencing the sensitivity of different viruses to interferon with emphasis on confirmation of the existence of separate resistance factors for RNA and DNA viruses; (c) the relation of delayed hypersensitivity and resistance of animals to virus infections, including the coordinate production of macrophage migration inhibitory factor and interferon in animals with delayed hypersensitivity inoculated with specific antigen.